Models of protein and amino acid requirements for cattle

Protein & Amino Acids - Ask the Dietitian®

The processes of protein synthesis and possibly of breakdown(turnover) require sources of dietary energy and are thus sensitiveto energy deprivation. Consequently, the energy balance of the bodybecomes an important factor in determining nitrogen balance andinfluences the utilization of dietary protein.

Protein needs for the athlete or body builder may be higher

The magnitude of the basal energy needs and of the total amountof protein turned over in a day are both related to active tissue mass(). Moreover, in young animals and growing children bothrates per unit of active mass are increased compared with thoseobserved in adults (). Nevertheless, as discussed in the nextsection, it has not proved possible to establish a constant numericalrelationship, covering all age ranges, between BMR and eitherprotein requirement or obligatory nitrogen loss, although such arelationship has been assumed by previous committees ().

However, the evidence on which these observations were basedis of limited value. Firstly, most habitual diets derive 10–14% oftheir energy from protein. Thus, when energy intake rises, so doesprotein intake and also intakes of many of the nutrients associatedwith protein in foods, such as the B-group vitamins and traceelements. Secondly, it is obvious that many environmental factorswill influence any selected measure of health. Populations thatcharacteristically have higher levels of protein intake tend to liveunder healthier conditions, whereas those with habitually lowerintakes are much more likely to be exposed to parasitic andinfectious disease. These confounding factors make it extremelydifficult to attempt to draw causal relationships. Thirdly, there aremany different measures of health and wellbeing; the criteria aretherefore complex and cannot easily be used to set physiologicalrequirements for protein.

In the final analysis, one would wish to set protein allowances inaccordance with such characteristics as health, growth,development, and longevity. This was, in fact, the approach used byour predecessors at the end of the nineteenth and beginning of thetwentieth centuries—Voit, Atwater, Benedict, and Cathcart ().The majority view, with Chittenden dissenting, appears to have beenthat protein intakes well in excess of physiologically determinedrequirements were associated with active and healthy lives.

Accordingly, recent studies have attempted to assess requirementsby using several levels of intake that encompass the expected rangeof requirements. This is one of the reasons why most estimates ofrequirements based on contemporary studies are higher than thosebased on data reported in the past. In addition, other variations inexperimental design contribute to the differences, such as the levelof dietary energy intake and physical activity. In the earlier studies,energy intake was intentionally increased to ensure weightmaintenance at low levels of protein intake. However, it is knownthat this results in more positive (less negative) N balances andtherefore lowers the apparent protein requirement () (section5.3).

The influence of energy balance on N balance extends fromsuboptimal up to excess levels of energy intake, so that any changein energy intake above or below the subject's needs is likely toinfluence his N balance, the effect being of the order of 1–2 mg ofN retained per kcalth added (0.24–0.48 mg of N per kJ) (). Thishas important implications for the determination of proteinrequirements when N balance is used as the criterion of adequacy(). In view of the difficulty of determining the energy needs ofindividual experimental subjects, this effect of energy intake must becarefully considered when assessing estimates of proteinrequirements obtained by the N-balance method.